But this evening I discovered just how poorly designed it is. Suppose your default printer in Windows is a network printer, in my case a home multifunction shared over plain old SMB (the default Windows printer sharing). Supposed said printer and the computer it’s attached to are offline, because you have no reason to turn them on right now. Try opening e-tax.

You get the delightful message “access violation at address 0055F9C0 in module ‘etax2006.exe’. Read of address 00000008.”

Solution: change your default printer, or turn on the network printer. Cos, you know, you absolutely must have a printer ready and raring when you type in your tax file number.

A problem is preventing Windows from accurately checking the licence for this computer. Error code 0x80090019

There are, it seems, a bunch of different things that can cause this problem, all of them related to Windows’ (in)ability to load the default cryptography DLLs with which it checks your registration details (every time you boot, apparently). One cause mentioned by the MS Knowledge Base article is your drive letters getting stuffed up, and another is if HKEY_USERS\.DEFAULT\Software\Microsoft\Cryptography is somehow damaged. But my drive letters hadn’t changed and I didn’t even have that registry key to begin with.

The simplest of the other suggestions that I could find was to boot Safe Mode and run System Restore — which I, in my infinite foresight, had turned off. Some folks spoke of running Windows Product Activation one more time, and the last-ditch fix is to reinstall completely. Obviously those weren’t particularly palatable options.

But then this article made me realise that the problem might, in fact, lie in HKEY_LOCAL_MACHINE\Software\Microsoft\Cryptography. So I took a known working copy of that key from my other PC, and applied it to the broken one in Safe Mode. My suspicion of borkedness was promptly confirmed — after a reboot, everything was fine.